Chris Wells wrote:
Fred - I would argue that you can see significant harmonics in comercial
buildings in the USA.
I guess I didn't make myself very clear. I did not mean to say that a 3 phase
load was required to produce harmonics. Quite the contrary. The harmonics are
created in unbalanced loads in three phase systems. A three phase motor, like
what would be found in an air-conditioning system would be a balanced load. A
switching power supply across one leg of a three phase system (system being
the operative word) is a harmonic generator. The harmonics are propagated
through the transformers (particularly open delta) and along the neutral
(stinger) in a wye. Non-linearites in single phase 240/120 systems can cause
harmonics too. This is usually seen when the transformer is operating at near
or full capacity.
Single phase power supply loads without Power Factor Correction front
ends
can create 3rd harmonics ~ equal in size to the fundamental.
The higher order harmonics dropping off in magnitude in a sinx/x
function.
The use of PFC front ends on PCs in the USA should be curbing much of
this.
This may or may not be the case. Some PFCs actually make the system efficiency
and harmonics worse! That is because a PFC will try and correct on its input
side as well as it's load side. For example lets say you have two loads with a
PF of 0.9. One of the loads has a PFC and the other one doesn't. The PFC will
correct its own load and the other (no PFC) load as well. Instead of
decreasing current (and ohmic losses) it actually increases current (over a no
PFC system). The further the uncorrected load is from the PFC the worse the
problem.
Fred Townsend
DC to Light
I suspect that the higher frequency mag field exposure in the office
enviroment is low.
Chris
----- Original Message -----
From: Fred Townsend <mailto:[email protected]>
To: Tang, George <mailto:[email protected]>
Cc: John Woodgate <mailto:[email protected]> ;
[email protected]
Sent: Tuesday, July 29, 2008 4:02 PM
Subject: Re: Another Cancer Scare?
George, thank you for your citations. Someone once wrote, "If
you want to
understand what is going on, follow the money." When I Google your citations
the ads from lawyers to join class action lawsuits on cell phones pop up. So
far what I read doesn't pass the smell test.
I have personally researched harmonics on power systems. They
are not that
common. They typically occur with 3 phase systems where switching power
supplies are used which means computer centers and the like. Computer centers
are a controlled environment where they try to keep the people away so they
won't pollute the computers. Commercial power lines are almost always in
conduit which means they are shielded as well. After all computer rooms are
full of magnetic media. (You wouldn't want to damage the media would you?) You
really have to work at finding low frequency harmonics that impinge on people.
By definition harmonics are smaller than the fundamental which
means if you
start with a small fundamental the harmonics are going to be even smaller. It
is common to increase the dosage to accelerate cancer testing so why not use
strong fields to accelerate the test. This is easy to do in the lab but
difficult to do if you are using observation data from real life. Of course if
you are using real life data, it is no longer a controlled environment.
There is good money to be made if you can claim to be an expert
witness that
has seen cancer cells caused by magnetic fields. It is not surprising then
that people will stand up in court and perjure themselves. In court, experts
can give opinion in place of facts which is something ordinary people can't
do. In the case of juries isn't it interesting that people that know nothing
about the subject at hand are given the final say.
Fred Townsend
DC to Light
Tang, George wrote:
John,
You have provided very good info. My comment is that
the fields used in
the experiments are relatively weak, in the range of a few microT to 1800
microT, and the durations are relatively short (a few days). Researchers have
found that when they increase the field strength up to 100 milli-G for months
up to 1 year, the results show an increase in cancer cells growth.
George
-----Original Message-----
From: [email protected] [mailto:[email protected]] On
Behalf Of John
Woodgate
Sent: Friday, July 25, 2008 11:39 PM
To: [email protected]
Subject: Re: Another Cancer Scare?
In message
<[email protected]>
<mailto:[email protected]> ,
dated Fri, 25 Jul 2008, "Tang, George"
<[email protected]>
<mailto:[email protected]> writes:
There has been some research published in the
last few years about
power line harmonics inducing cancer in the
human body. These are
fields in the kHz to hundreds of kHz and
currents in the range of tens
of amps to hundreds of amps.
There has also been a very great deal more research
published that finds
no such effects. See, for example:
http://www.faqs.org/faqs/medicine/powerlines-cancer-faq/
If there were hundreds of amps at hundreds of kHz
circulating in any
power distribution system, it would collapse due to
system capacitor
heating and vast over-voltages on system inductors.
--
OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and
www.isce.org.uk
Either we are causing global warming, in which case we
may be able to stop
it,
or natural variation is causing it, and we probably
can't stop it. You
choose!
John Woodgate, J M Woodgate and Associates, Rayleigh,
Essex UK
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